A multichannel shot noise approach to describe synaptic background activity in neurons

Systems driven by Poisson-distributed quantal inputs can be described as “shot noise” stochastic processes. This formalism can apply to neurons which receive a large number of Poisson-distributed synaptic inputs of similar quantal size. However, the presence of temporal correlations between these inputs destroys their quantal nature, and such systems can no longer be described by classical shot noise processes. Here, we show that explicit expressions for various statistical properties, such as the amplitude distribution and the power spectral density, can be deduced and investigated as functions of the correlation between input channels. The monotonic behavior of these expressions allows an one-to-one relation between temporal correlations and the statistics of fluctuations. Multi-channel shot noise processes, therefore, open a way to deduce correlations in input patterns by analyzing fluctuations in experimental systems. We discuss applications such as detecting correlations in networks of neurons from intracellular recordings of single neurons.     

On the hydrodynamical description of gauge theories

Classes of gauge equivalent configurations are characterized in terms of local gauge invariant quantities. The construction of such quantities is performed for scalar electrodynamics and theories of SU(2)-gauge fields in two different representations. Topologically nontrivial configurations are discussed.     

Determination of the rate of production and dissolution of biosilica in marine waters by thermal ionisation mass spectrometry

A new method is described for a precise and simultaneous determination of the rate of production and dissolution of biosilica in marine waters, using isotopic dilution technique. No HF or F₂ is required for chemical preparations as the change in isotopic composition is measured on silica producing SiO₂⁻ ions. The seawater sample flask is spiked with ³⁰Si(OH)₄ (<10% of increase in situ concentration) and incubated in in situ conditions. At the end of incubation, changes of the ³⁰Si:²⁸Si ratios in particulate and liquid phases are measured by using a thermal ionisation mass spectrometer Finnigan THQ. The relative analytical precision of the isotopic ratio measurements is <0.5%. The limit of detection of the change in isotopic ratio during incubation is 0.02 atom%. The overall repeatability determined on eight subsamples (average production: 0.23 μM day⁻¹; average dissolution: 0.07 μM day⁻¹) is ±0.02 and ±0.01 μM day⁻¹ for production and dissolution, respectively. Using mass and isotopic balances of the particulate and dissolved phases in the incubation flask, the best estimates for production and dissolution rates are calculated iteratively. This method was applied to 112 samples of marine waters (production, range: 0.00-2.38 μM day⁻¹; dissolution, range: 0.00-1.18 μM day⁻¹).     

Gold‐Catalyzed Regiospecific C−H Annulation of o‐Ethynylbiaryls with Anthranils: π‐Extension by Ring‐Expansion En Route to N‐Doped PAHs

We describe a novel, short, and flexible approach to diverse N‐doped polycyclic aromatic hydrocarbons (PAHs) through gold‐catalyzed π‐extension of anthranils with o‐ethynylbiaryls as reagents. This strategy uses easily accessible starting materials, is simple due to high step and atom economy, and shows good functional‐group compatibility as well as scale‐up potential. Mechanistically, the tandem reaction is proposed to involve a nucleophilic addition/ring opening/regiospecific C?H annulation/protodeauration sequence terminated by a Friedel–Crafts‐type cyclization. Photophysical studies of the products indicated violet‐blue fluorescence emission with quantum yields up to 0.45.     

Femtosecond autocorrelation measurements based on two-photon photoconductivity in ZnSe

Two-photon photoconductivity in ZnSe is used to record femtosecond autocorrelation functions. This technique requires <100 muW of average power of a typical mode-locked femtosecond Ti:sapphire laser and distinguishes itself by a dynamic range over several decades and great conversion bandwidth, permitting the sensitive correlation of pulses of a few femtoseconds.     

The rapid separation of trace metals from neutron-activated saline matrices by chelation on a poly-5-vinyl-8-hydroxyquinoline column

Instrumental neutron activation analysis of biological and environmental samples suffers from interferences caused by high salt concentrations. Poly-5-vinyl-8-hydroxy-quinoline coated on controlled pore diameter glass beads is suggested as a chelating column for the rapid removal of aluminum, vanadium, copper and manganese from neutron-activated sea-water samples. Separation from bulk elements is satisfactory at flow rates of 20 ml min⁻¹. With addition of carriers and with chemical yield determinations, relative standard deviations of 2–10% can be achieved for spike concentrations of 0.1 μg Mn ml⁻¹, 0.3 μg V ml⁻¹, 20 μg Al ml⁻¹ and 1.0 μg Cu ml⁻¹.     

Preparation and properties of carboxyl-functional mixed esters of hydrolyzed starch

Mixed esters of hydrolyzed starch represent a new class of chemically modified natural polymers demonstrating a broad range of properties. Members of this class of polymers have both neutral aliphatic ester side chains and carboxyl-functional half-ester side chains. Use of hydrolyzed starch as the backbone polymer results in products that are considerably lower in molecular weight than whole starch derivatives, but which are still polymeric in character. Synthesis proceeds smoothly in pyridine using anhydrides as acylating agents and the pure solid mixed ester products are isolated by precipitation in water. Measurement of degree of substitution (D.S.) by NMR or hydrolysis characterizes the chemical composition of the polymers. The actual D.S. achieved in the synthesis depends upon the competition between starch and residual water for anhydrides, which can be quantitatively evaluated by monitoring the acid content of the reaction mixture. The T_g and T_s of starch mixed esters vary with both D. S. and length of the aliphatic ester side chain. As the composition changes from acetate—phthalate to butyrate—phthalate a T_g range of greater than 100°C is observed. Hydrolyzed starch mixed esters are hydrophobic and organic-soluble, but may be readily solubilized in aqueous base through the half-ester groups. Solutions show surface activity which varies according to the type and extent of substitution.     

Synthesis of hindered tertiary amines by a mild reductive amination procedure

An efficient metal-free procedure for the synthesis of tertiary amines by the reductive amination of carbonyl compounds is reported, which allows a convergent access to sterically hindered amines of the general formulas NR(R′)₂ and NRR′R″. The mild and operationally simple protocol uses the Hantzsch ester for transfer hydrogenation and catalytic amounts of thiourea for imine activation.     

The determination of sulfide in the aqueous environment

Direct measurements were made by use of a sulfide ion-selective electrode in conjunction with a double-junction reference electrode and an Orion Model 407 A/F specific ion meter. This method enables simple and rapid determinations of the total sulfide ion concentration in water. Total sulfide was determined in samples collected from lakes in Fort Bend County. Texas, which ranged from 4 ppb to 18 ppb and from the Houston Ship Channel which ranged from 13 ppb to 42 ppb.A series of experiments shows that (1) the rate of oxidation of S^2? exposed to air is about three times slower for solutions containing the sulfide antioxidant buffer (SAOB II) and (2) the rate of oxidation of S² in tightly capped containers is about twice as fast for solutions not containing SAOB II than for solutions containing SAOB II.It was concluded that the use of SAOB II almost eliminates the problem of air oxidation of sulfide ions during direct measurements and that sulfide ion concentrations (as low as 4 ppb) can be determined directly in the laboratory and in the field by use ofthis electrode method.